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Basic Solid State Relays

Basic Solid State Relays

These Relays would be ideal for applications where many relays are needed and the load current requirements are low. Due to their small size a large number of relays could be mounted on a single printed circuit board. The relays are based on a 4N33 Optoisolator package. This device has a Darlington output transistor and as the first figure shows it can be used alone for low power applications of up to 30Ma.
The current required to operate these relays is significantly less than for mechanical relays types. Also these devices may be less expensive than mechanical relays in many instances as Optoisolators often cost less than a dollar each.


The control and load supply voltages for the examples is 12 Volts, other voltages can be used with a corresponding adjustment to resistors in the circuit. For example if the voltages are doubled then the value of the 1K and/or 470 Ohm resistors should also be doubled in order to keep the currents through the LED and base of Q1 at the same level.

If the 2N3906 transistor is replaced with Darlington type, larger currents can be handled, 1 Amp and more, but the voltage drop across the relay increases and the cost of each unit rises. The diode D1, at the output of FIGURE 2 is a 1N4001. This is an anti-ringing diode and is only really needed if there is an inductive load such as a solenoid being controlled.

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